Maintenance Agents Flashcards
1
Q
What is inhalation anaesthesia?
A
- Induction: Usually with an injectable and can be with a gas
- Maintenance: Usually with an inhalational agent and can with injectable
- For both: Part of the gas provided in nitrous oxide or a liquid vaporised into the mixed gas
2
Q
What are volatile agents?
A
- Liquid anaesthetic agents that are vaporised into gases, which are then inhaled by the patient
- A vaporiser is a device that adds volatile agent to a carrier gas
3
Q
What are the ideal properties of volatile agents?
A
- Chemically stable
- Non-flammable
- No environmental hazard
- Inexpensive
- High potency
- Low blood solubility
- Minimal metabolism
- Minimal toxicity
- No cardiorespiratory side effects
- Analgesia
- Easily administered
4
Q
What are the advantages of inhalation agents?
A
- Administered in oxygen through ET tube
- Rapid action
- Change in depth of anaesthetic rapid
- Rapid recovery
5
Q
What are the disadvantages of inhalation agents?
A
- Anaesthetic machine costly and requires full understanding by user
- Workplace pollution
- Dose dependent cardiopulmonary depression
6
Q
What are the pharmocokinetics of inhalation agents?
A
- Agent is inhaled and passed into lungs
- Diffuses into the pulmonary circulation
- Travels around the body via the circulatory system and is distributed to the tissues
- Depending on agent solubility, it will diffuse back into the blood and be eliminated by the lungs
7
Q
What are the factors that affect gaseous uptake?
A
- Inspired concentrations of the agent
- Alveolar concentration
8
Q
How do inspired concentrations affect gaseous uptake?
A
- Vaporiser setting: the higher the % on the dial, the more concentrated the volatile agents
- FGF: a high FGF will affect the volume of volatile agents in a rebreathing system
- Function of the breathing system: rebreathing systems are bulky and have lots of components meaning that it takes longer for higher concentrations to be reached
9
Q
What is alveolar concentration?
A
- This is the partial pressure
- Used as a blood: gas partition coefficient
- Reliant on good alevolar ventilation and pulmonary circulation
10
Q
What is the blood : gas partition coefficient?
A
- The measure of the distribution of the inhalation agent between the blood and gas phases within the body
- Low solubility: does not dissolve in the blood so rapid induction and recovery
- High solubility: dissolves more readily in the blood, so slow induction and recovery
11
Q
What is potency?
A
- Is expressed as the minimum alveolar concentration (MAC value)
12
Q
What is a MAC value?
A
- The amount of volatile gas that is needed to keep 50% of patients asleep when exposed to a standard noxious stimuli
13
Q
What increases the MAC value?
A
- Hyperthermia
- Catecholamines
- Hyperthyroidism
- Hypernatremia
14
Q
What decreases the MAC value?
A
- Hypothermia
- Hypoxaemia
- Hypercapnia
- Drugs that affect CNS
- Analgesia agents
- Pregnancy
- Old age
- Hypotension
- Hypothyroidism
15
Q
What factors affect the elimination of inhalants?
A
- CNS concentrations low enough, the patient will start to regain consciousness
- Newer gases almost 100% eliminated through exhalation
- Older gases 25% is metabolised
- Removal of agent is determined by B : G pCE
- Can speed up elimination in rebreathing system by turning off the gas and increasing the oxygen flow
16
Q
What is the inhalant effect on the cerebrum?
A
- Decrease metabolic rate
- Decrease in cerebral oxygen concentration
- Decrease in cerebral blood flow
-Cerebral vasodilation - Increase in blood volume
17
Q
What is the inhalant effect on the cardiovascular system?
A
- CV depression is a major side
- This is dose dependent
-Cardiac output, systemic vascular resistance and arterial BP, due to decreased myocardial contractility and vasodilation
18
Q
What is the inhalant effect on the respiratory system?
A
- Dose dependent respiratory depression
- Could lead to hypercapnia
- Decrease in minute volume
- Decrease in respiratory rate
- Decrease in tidal volume
19
Q
What is the inhalant effect on the renal and hepatic system?
A
- Perfusion is decreased due to hypotension
- Renal perfusion issues may lead to decreased urine output
- Sevoflurane with rebreathing systems can react with soda-line, which makes compound A (is nephrotoxic in rats)
- Decreased perfusion to the liver may lead to decreased drug metabolism
20
Q
What is halothane?
A
- Very potent halogenated hydrocarbon
- Requires preservative
- Does not decompose in presence of warm soda lime
- Respiratory and cardiovascular depressant
- Metabolised by liver (20-25%)
21
Q
What is isoflurane?
A
- Fluorinated ether
- Very potent
- Delivered using a precision out of circuit vaporiser
- Strong pungent colour
- Respiratory and cardiovascular depressant
- Low solubility
- Under 1% metabolised by patient
22
Q
What is sevoflurane?
A
- Fluorinated ether
- Slightly less potent than isoflurane
- Requires different vaporiser to isolflurane
- Metabolised to produce fluoride ions
- Less soluble than isoflurane
- Cardiovascular and respiratory depressant
- 3% metabolised
23
Q
What is desflurane?
A
- Fluorinated ether
- Needs specialised vaporiser
- Expensive
24
Q
What is nitrous oxide?
A
- Gaseous inhalant
- Used in combo with more potent inhalant such as halothane
- Low solubility so rapid transfer
- Cannot achieve true anaesthesia alone as MAC values are high
25
What are the side effects of using nitrous oxide?
- Mild sympathetic stimulation
- Diffuses into gas filled spaces
- Diffusion hypoxia, so adequate oxygen required during recovery
26
What is the nitrous oxide second gas effect?
- Nitrous oxide will increase the rate of uptake of other inhalational agents into the blood stream and therefore an appropriate level of anaesthesia will be reached more quickly
27
What is nitrous oxide diffusion hypoxia?
- Caused by rapid movement of gas
- When recovering, nitrous oxide should be switched off and and oxygen should be maintained
- if not, oxygen can reach dangerously low levels in alveoli as nitrous diffuses out of blood stream
28
What is occupational exposure?
- Gas leaking into the room, due to disconnection of the patient and not flushing out the anaesthetic circuit, poorly fitted mask, underinflated ETT cuff and during recovery from GA
- Can causes mutagenic, carcinogenic and teratogenic effects
29
What are the inhalational exposure issues?
- Spontaneous abortion
- Fertility issues
- Minor congenital abnormalities
- Leukaemia and lymphoma
- Liver/renal disease
- Effects on the immune system
